In some situations, such as MAK, OEM or Office 365 keys, Microsoft has stored then in a way no one's been able to decrypt yet, or they don't store it at all, and only use the Product ID for support, etc. No software we know of, including Newt, can retrieve the entire license key in these situations. Microsoft allows the last 5 characters to be read in order to help match it up with keys from the original purchase, and for auditing purposes. However retail license keys should be fully readable.
If a method's developed to read keys like this, we'll definitely add it.